1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium, particularly the magnetic recording medium using a thin film of ferrite, as well as the improved films of ferrite for to recording medium.
2. Description of the Prior Art
It is generally known that as a recording medium, in addition to the above-mentioned thin film of ferrite, a coating film of acicular crystalline particles of .gamma.-Fe.sub.2 O.sub.3 or an electrolytically deposited film of an Ni-Co alloy can also be used.
The coating film is polished in order to make the film thinner and to increase the recording density of the produced recording medium. However, it is considered quite difficult to further increase the recording density, because of the difficulty in making the film thinner by polishing and because of the fact that the .gamma.-Fe.sub.2 O.sub.3 crystals are dispersed in their binder.
The magnetic recording medium of the Ni-Co alloy can be easily provided in the form of a thin film. However, this alloy is corrosive and exhibits poor wear resistance. Therefore, it is necessary to apply a protective film with a thickness of 0.2 microns on the Ni-Co alloy, with the result being that the distance between a magnetic head and the recording medium is increased according to the added thickness of the protective film. Because of the increased distance between the magnetic head and the recording medium the area for memorizing a unit of information is widened, thereby making it difficult to perform highly densified recording.
The known thin film of ferrite is free from the above-described disadvantages and is far more superior in corrosion resistance and wear resistance than either the coating film or the electrolytically deposited film.
The term "a thin" film of ferrite herein used indicates a continuous film of the iron oxides of Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3, which oxides are crystallized directly on a substrate for supporting the recording medium, such as a disc, so that the crystals remain in a continuous state on the substrate, i.e., the crystals are not separated by a binder. The film of ferrite is generally produced by the following steps.
An .alpha.-Fe.sub.2 O.sub.3 is continuously formed by using any of the procedures of reaction sputtering, i.e., chemical sputtering, the coprecipitation method, the vapor growth method and the like. The .alpha.-Fe.sub.2 O.sub.3 is then reduced to Fe.sub.3 O.sub.4, which is oxidized, if desired, to .gamma.-Fe.sub.2 O.sub.3. The .alpha.-Fe.sub.2 O.sub.3 can also be formed from Fe(OH).sub.3 or Fe(OH).sub.2, which is precipitated on the substrate. If the thin film of ferrite has a thickness of one micron or less, then this film can be produced without the polishing required for the coating film.
One of the problems residing in the known films of ferrite used as a recording medium is that it is difficult to stably reduce .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4, due to the narrow temperature range for the reduction. This reduction step is likely to bring about an excessive reduction of .alpha.-Fe.sub.2 O.sub.3 into a metallic iron.
Another problem residing in the known ferrites is that it is difficult to adjust the temperature for oxidizing the Fe.sub.3 O.sub.4 to .gamma.-Fe.sub.2 O.sub.3 in order to provide the .gamma.-Fe.sub.2 O.sub.3 with excellent magnetic properties.
Still another problem residing in the films of ferrite is that their magnetic properties are less than the desirable properties of the highly densified recording medium. Namely, although properties exhibiting a coercive force (Hc) from 400 to 500 Oe and a squareness ratio of 0.5 or more are desired, the known ferrites exhibit only 300 Oe of coercive force and 0.4 squareness ratio.